The field of the invention is that of electronic circuits. More precisely, the invention concerns the electrical power supply for such circuits and in particular the voltage of the supplied power.
Micro-electronic circuits are designed to operate at a nominal power supply voltage (for example VDC=5 V). They can certainly operate over a predetermined range of voltages around this nominal value, however their correct operation cannot be guaranteed outside this range. For example, a memory supplied with a too low voltage (for example VDC MIN=2.55 V) may exhibit random behaviour, leading to undesired read/write operations.
It is therefore necessary to monitor the supply voltage to the circuit and only permit it to operate when the supply voltage is within the desired range (for example VDC greater than 2.55 V). In this way the circuit will only operate when it is actually capable of operating without causing an error. Below the nominal supply voltage the circuit is forced into RESET mode.
This is the role of the power supply controller.
As is well known, in order to know precisely the supply voltage VDC supplied to a circuit, a part of this voltage VDC is generally compared at the output of a voltage reference module in accordance with the principle illustrated in FIG. 1.
The bandgap module 11 is based on pnp transistors which output a precise reference voltage Vref (for example Vref=1.25 V). A voltage divider 12, formed from two resistors R1 and R2 (for example each of 100 kxcexa9) outputs a fraction of VDC (in the described example: VDC/2). These two voltages are sent to a comparator 13 which triggers a RESET command whenever VDC/2 less than Vref. A RESET is therefore obtained whenever VDC/2 less than 2.5 V.
A problem with this technique is that while it is effective for slow voltage rises, it does not work for rapid voltage rises. In fact, in this case, the output of the bandgap module can remain at 0 V even though the supply voltage VDC has already reached 2 V, for example. The comparator 13 triggers the RESET command even though the supply voltage has still not reached the desired nominal value.
In order to alleviate this problem, we considered adding to the controller a RC circuit 21 for the supply voltage VDC as shown in FIG. 2. This RC circuit forces a RESET command for rapid rises while waiting for the device in FIG. 1 to trigger.
However, this technique is not 100% reliable. Depending on the slope of the voltage rise, the supply voltage VDC and the temperature and/or the technology used, the RC circuit 21 may trigger the RESET command while the part of the detector shown in FIG. 1 is no longer operating.
In addition, this technique is not suitable for circuits using thin film technology (0.35 xcexcm for example) which does not support a supply voltage greater than a predetermined threshold (for example 4 V).
Therefore, if we consider a circuit that must be able to operate with a battery voltage (VBAT) of between 2.5 and 5.5 V, while the technology cannot support more than 4 V, an internal regulator is connected (from a bandgap module), which will supply the remainder of the circuit with 3 V. However, all the circuits connected to VBAT must then be built with transistors that can withstand 5.5 V. It must therefore use a thicker oxide layer (for example 0.6 xcexcm) which is less efficient. This applies in particular to the bandgap module and the 3 V regulator.
This implies that the bandgap module only operates from a minimum voltage of 2.4 V, for example, while with 3 V transistors, it would operate from 1.6 V. With such a device, in the case of a slow rise in voltage to 2.3 V, the RC circuit 21 triggers the RESET command while the bandgap module is still at 0 V, since it receives a lower voltage (2.3 V) than its minimum operating voltage (2.4 V).
Again the comparator 12 triggers the RESET command (since it sees VDC/2=1.15 V greater than 0 V), while the supply voltage VDC has not risen to 2.55 V.
In addition, using this technique, it is not possible to control voltages of less than 2.4 V since the bandgap module no longer operates.
The invention has the particular objective of alleviating the problems of current technology.
More precisely, one objective is to provide a power supply controller for electronic circuits that operates efficiently and reliably to prevent the operation of a circuit until the supply voltage has reached a threshold value, under all conditions, and in particular whether the voltage rises are slow or rapid.
Another objective of the invention is to provide such a power supply controller that is simple, inexpensive and easy to build and implement. In particular, it is an objective of the invention to provide such a controller, all of whose constituent parts can be built using the same technology, in particular thin film technology.
The invention also has the objective of providing such a power supply controller that can work at low voltages compared with earlier technology (and for example less than 2.4 V).
These objectives, as well as others that will be revealed later, are achieved by using a power supply controller for an electronic circuit providing a supply voltage (VDC) and preventing the operation of the circuit by using a RESET signal when the supply voltage is below a first predetermined threshold, the controller comprising a first comparator (C2) comparing a voltage proportional to supply voltage with a reference voltage and activating the RESET signal when the voltage proportional to the supply voltage is lower than the reference voltage and a bandgap module that provides the principal reference voltage (VBGAP).
According to the invention, the controller includes preliminary reference devices, immediately supplying a preliminary reference voltage (V09), that is lower than the principal reference voltage, but fairly accurate, and control circuits receiving the preliminary reference voltage (V09) and the principal reference voltage (VBGAP) and automatically activating the RESET signal for as long as the principal reference voltage (VBGAP) has not reached a second predetermined threshold.
Thus, as long as the principal reference voltage is not available and whether the voltage rise is slow or rapid, means are available to ensure that the RESET command is triggered and that therefore there will be no random operation of the circuit due to the supply voltage being too low.
It is preferable that the means of control should include means for selecting a reference voltage (VREF) for the first comparator (C2) between the preliminary reference voltage (V09) and the principal reference voltage (VBGAP), the preliminary reference voltage (V09) being selected for as long as the principal reference voltage (VBGAP) has not reached the second predetermined threshold.
The power supply controller also includes a regulation module providing a regulated supply voltage (VDC) to the circuit which takes advantage of the reference voltage (VREF).
According to an advantageous aspect of the invention, the regulated supply voltage CVDC) also supplies the bandgap module. This permits the latter to be built using thinner technology and to operate at lower voltages.
According to another advantageous characteristic of the invention, the control circuits issue a command to the regulation device, controlling the means of amplification of the regulated supply voltage (VDC) at a third predetermined threshold, until the principal reference voltage reaches the first predetermined threshold.
The gain is therefore temporarily increased while using the preliminary reference voltage to take account of the fact that it is lower and nevertheless obtain an acceptable supply voltage.
In accordance with a preferable mode of operation, the preliminary reference devices include a transistor connected as a diode.
Advantageously, the control devices include a second comparator (C1) supplied from the preliminary (V09) and principal (VBGAP) reference voltages and supplying an inverter (INV) driving a transistor (TP1) provided to force the RESET signal.
Preferably, the means of selection include two transistors, receiving respectively the preliminary (V09) and principal (VBGAP) reference voltages and driven respectively by the output of the second comparator (C1) and the output of the inverter (INV).
According to an advantageous characteristic, the regulation devices include an amplifier (AOP) supplying the regulated supply voltage (VDC) and supplied partly from the reference voltage (VREF) and partly from a voltage divider to which the regulated supply voltage (VDC) is fed back.
Preferably, the limiting devices include a transistor connected so as to short circuit part of the voltage divider.
The invention also concerns electronic components and electronic devices including, or working with, at least one power supply controller as described above.